1. Field of the Invention
The present invention relates to a method of inspecting an exposure apparatus which exposes an object to be exposed such as a wafer through a mask pattern, a mask for inspecting an exposure apparatus, and an exposure apparatus.
2. Description of the Related Art
An exposure apparatus is known to form images with different characteristics on an object to be exposed, depending on the polarization state of the projection light. For example, for an exposure with two-beam interference using TM wave 111 (p-polarization) projection light or TE wave 112 (s-polarization) projection light, as shown in FIG. 14, the latter is known to provide a higher contrast interference wave. The term TM wave refers to light with its electric vector vibration direction being parallel to the plane of incidence. The term TE wave refers to light with its electric vector vibration direction being perpendicular to the plane of incidence.
The resist surface as the object to be exposed is known to have a light absorption rate higher for the TM wave incident light than for the TE wave incident light. In particular, TM wave with its incidence angle at the so-called Brewster's angle can provide the almost zero reflectance.
It has thus been attempted in the exposure apparatus to positively control the polarization state of the illumination light to change the imaging characteristics.
For example, Japanese Patent No. 3246615 discloses a method in which the illumination light is linearly polarized to form an image mainly with the TE wave. Japanese Patent No. 2836483 discloses a method in which the illumination light is linearly polarized to form an image mainly with the TM wave.
Optical elements of the illumination optical system may be contaminated, degenerated, or misaligned due to vibration, to change the polarization direction of the illumination light that has been set during the exposure apparatus assembly operation. Such a variation in the polarization direction of the illumination light can affect the imaging characteristics of the exposure apparatus. Before being used, such an exposure apparatus thus needs to be inspected for variation in the polarization direction, and the optical elements need to be appropriately cleaned depending on the inspection results.
In the conventional exposure apparatus, however, it has not been directly confirmed that the illumination light is not in the desired polarization state. In the conventional exposure apparatus, it is possible to detect the exposure latitude or depth of focus and the like from the exposure conditions of the resist and the like to determine that the desired imaging conditions are not provided. In the conventional exposure apparatus, however, it is not possible to directly determine the variation in the polarization direction, because various possible reasons other than the variation in the polarization direction of the illumination light may prevent the desired imaging conditions.